1. Field of the Invention
This invention relates to pressure storage (or common rail) fuel injection systems, in which high pressure fuel stored in pressure storage (or common rail) is injected into cylinders at predetermined injection timings.
2. Description of the Prior Art
In such a pressure storage fuel injection system, fuel is fed from a high pressure fuel pump to a pressure storage for storing pressure therein, and then injected through fuel injection valves into engine cylinders at injection timings predetermined through an electronic control or the like. This system is important in large size diesel engines for ships, and has recently become applied to diesel engines for small size, high speed vehicles (such as buses and trucks).
The pressure storage fuel injection system, unlike well-known jerk fuel injection systems, is free from the disadvantage of injection pressure reduction at low speed, that is, it permits high pressure injection to be readily realized at low speed as well. Thus, it has pronounced advantages in that it permits fuel cost reduction, output increase, soot reduction, etc.
FIG. 11 shows a prior art pressure storage fuel injection system used for vehicle exclusive engines.
Referring to this Figure, designated at 10 is a fuel injection valve assembly. The fuel injection valve assembly 10 has a nozzle 16 having a row of fuel injection ports 12 provided at the end and a fuel pool storing fuel supplied to the ports 12.
In the nozzle 16, a needle valve 18 is fitted slidably for controlling the communication of the fuel pool 14 and fuel injection port 12 with each other. The needle valve 18 is always biased in the closing direction by a spring 24 via a push rod 22 which is accommodated in a nozzle holder 20. In the nozzle holder 20 a fuel chamber 26 is defined. In the fuel chamber 26 is slidably fitted a pressure application piston 28 which is coaxial with the needle valve 18 and push rod 22.
The fuel chamber 26 is communicated through a uni-directional valve 30 and an orifice 32 parallel therewith with a first outlet line b of a three-way electromagnetic valve 34. The electromagnetic valve 34 has an inlet line a communicating with a pressure storage 6 and a second outlet line c communicating with a fuel tank 38. The first outlet line b is selectively communicated with the inlet line a or the second outlet line c by a valve body 42 which is driven by an electromagnetic actuator 40. When the electromagnetic actuator 40 is de-energized, the inlet line a is communicated with the first outlet line b. When the actuator 40 is energized, the first outlet line b is communicated with the second outlet line c. In the nozzle holder 20 and nozzle 16, a fuel line 44 is provided which communicates the fuel pool 14 with the pressure storage 36.
Fuel under a high pressure predetermined in advance according to the engine operating condition is supplied to the pressure storage 36 by the high pressure fuel pump 46. The high pressure fuel pump 46 has a plunger 50 which is driven for reciprocation by an eccentric ring or cam 48 driven in an interlocked relation to the engine crankshaft. Fuel which is supplied from a fuel tank 38 to pump chamber 54 in the pump 46 is pressurized by the plunger 50 to be pumped out through a uni-directional valve 56 to the pressure storage 36.
A spill valve 64 is provided between a discharge side line 58 leading from the pump chamber 54 of the high pressure fuel pump and a withdrawal side line 60 leading to the feed pump 52. The spill valve is on-off operated by an electromagnetic actuator 62. The electromagnetic actuator 62 and the electromagnetic actuator 40 of the three-way electromagnetic valve 34 are controlled by a controller 66.
The controller 66 controls the electromagnetic actuators 40 and 62 according to output signals of a cylinder discriminator 68 for discriminating the individual cylinders of multi-cylinder engine, an engine rotation rate/crank angle sensor 70, an engine load sensor 72 and a fuel pressure sensor 74 for detecting the fuel pressure in the pressure storage 36, as well as, if necessary, such auxiliary information 76 as detected or predetermined input signals representing atmospheric temperature and pressure, fuel temperature, etc. affecting the engine operating condition.
Briefly, the pressure storage fuel injection system having the structure as described operates as follows.
The plunger 50 of the high pressure fuel pump 46 is driven by the eccentric ring or cam 48 which is driven in an interlocked relation to the engine crankshaft, and low pressure fuel supplied to the pump chamber 54 by the feed pump 52 is pressurized to a high pressure to be supplied to the pressure storage 36.
According to the engine operating condition, the controller 66 supplies a drive output to the electromagnetic actuator 62 for on-off operating the spill valve 64. The spill valve 64 thus sets a predetermined pressure (for instance 20 to 120 MPa) as fuel pressure in the pressure storage 36.
Meanwhile, a detection signal representing the fuel pressure in the pressure storage 36 is fed back from the sensor 74 to the controller 66.
The high pressure fuel in the pressure storage 36 is supplied through the fuel line 44 of the fuel injection valve 10 to the fuel pool 14 to push the needle valve 18 upward, i.e., in the opening direction. In the meantime, when the fuel injection valve 10 is inoperative, the electromagnetic actuator 40 for the three-way electromagnetic valve 34 is held de-energized, thus having the inlet a and first outlet b in communication with each other. In this state, high pressure fuel in the pressure storage 36 is supplied through the uni-directional valve 30 and orifice 32 to the fuel chamber 26.
At this time, the pressure application piston 28 in the fuel chamber 26 is held pushed downward by the fuel pressure in the chamber 26, and a valve opening force which is the sum of the downward pushing force of the fuel pressure and the spring force of the spring 24 is being applied via the push rod 22 to the needle valve 18. The needle valve 18 is thus held at its closed position as illustrated because the area, on which the fuel pressure acts downward on the pressure application piston 28, is set to be sufficiently large compared to the area, on which fuel pressure acts upward on the needle valve 18, and further the downward spring force of the spring 24 is acting additionally.
When the electromagnetic actuator 40 is energized by drive output of the controller 66, the communication between the inlet line a and first outlet line b is blocked and, instead, the first outlet line b and second outlet line c are communicated with each other, thus communicating the fuel chamber 26 through the orifice 32 and second outlet line c with the fuel tank 38 and removing the fuel pressure having acted on the pressure application piston 28. The upward fuel pressure acting on the needle valve 18 thus comes to surpass the spring force of the spring 24, thus opening the needle valve 18 to cause injection of high pressure fuel from the fuel pool through the fuel injection port 12 into the cylinder.
After the lapse of a predetermined period of time set according to the engine operating condition, the controller 66 de-energizes the electromagnetic actuator 40, whereupon the inlet line a and first outlet line b of the three-way electromagnetic valve 34 are communicated again with each other, causing the fuel pressure in the pressure storage 36 to be applied to the pressure application piston 28. As a result, the needle valve 18 is closed, thus bringing an end to the fuel injection.
The optimum fuel injection pressure for engine performance of the above pressure storage fuel injection system, will now be considered.
(1) Under low load, the high pressure injection deteriorates the fuel consumption (i.e., fuel consumption rate). This means that it is necessary to provide high pressure injection under this condition.
Under high load, it is necessary to provide high pressure injection for the purposes of reducing the soot generation and reducing the exhaust gas particulation.
(2) Setting the high pressure injection over the entire engine operating condition leads to engine noise increase due to increase of the initial combustion (i.e., preliminary air-fuel mixture combustion).
From the standpoint of suppressing the engine noise, the fuel injection pressure is desirably made as low as possible to an extent having no adverse effects on the exhaust gas state and fuel cost, and the fuel injection pressure during idling and under low load of the engine is adequately about 20 to 30 MPa.
From the above technical standpoints, the prior art pressure storage fuel injection system shown in FIG. 11 has the following problems.
A. When high pressure injection under low load is quickly changed to high load such as when quickly accelerating the vehicle, a certain time is taken until the pressure storage pressure increases to the requested level. Due to this delay in the pressure increase response, it is impossible to inject a large amount of fuel while holding the low pressure fuel injection, and the desired amount of fuel can not be injected, thus resulting in engine output shortage at the time of transient operation requiring quick acceleration.
In the prior art pressure storage fuel injection system, as shown in FIG. 14, during idling the common rail pressure (i.e., pressure in the pressure storage) has to be controlled to 20 MPa for reducing noise and ensuring smooth rotation. Under a low load engine operating condition, the pressure has to be controlled to 30 to 40 MPa for preventing fuel cost deterioration. Further, under a high load engine operating condition the pressure has to be controlled to 80 to 120 MPa for reducing soot generation and particulation. With such structure where the common rail pressure is varied in the above way, however, when the pressure storage pressure is quickly increased from low pressure injection (for instance under 20 MPa) under low load to high pressure injection (for instance 90 MPa) under high load, a delay is generated in the common rail pressure increase from 20 MPa to 90 MPa, thus causing the fuel injection during the open state of the needle valve to be less than the injection under predetermined pressure. Consequently, the engine output during the quick acceleration becomes less than the predetermined engine output. For example, as shown in FIG. 15, the instantaneous engine torque during the engine acceleration becomes greatly lower than the engine torque with the conventional row fuel injection pump.
The lines (a) to (c) in FIG. 15 show a relation between the engine crankshaft torque and the engine rotation rate, with the line (a) showing the relation obtained with a prior art pressure storage fuel injection system, the line (b) showing the relation obtained with a well-known row fuel injection pump, FIG. 15 and the line (c) showing the relation obtained with a pressure storage fuel injection system to be described later according to the invention.
B. To preclude the above drawback, the valve opening time of the fuel injection valve of the pressure storage fuel injection system may be prolonged to maintain the desired fuel injection. In such a case, however, the fuel injection is increased in the low pressure injection, thus resulting in the increase of black soot and particulation in the exhaust gas.
C. In connection with the above problems A and B, with the prior art common rail fuel injection system the instantaneous engine torques at intermediate and low engine rotation rates during quick acceleration of the engine are very low compared to the case of the well-known row fuel injection pump under the assumption that the maximum engine output is equal. Therefore, the acceleration character of the vehicle is greatly reduced.
To solve this problem, there is a fuel injection system which has been proposed as an invention disclosed in Japanese Patent Laid-Open Publication No. 93936/1994. In this system, two common rails (i.e., pressure storages), that is, a high and a low pressure side common rail system, are provided for switching one over to the other in dependence on the engine operating condition.
However, such a fuel injection system having the high and low pressure common rails requires, correspondingly two different, i.e., high and low pressure, fuel injection systems. Such a system is complicated in construction and increased in size so that its mounting in a vehicle engine encounters difficulties.
In the meantime, in diesel engines the fuel supply in one combustion cycle is made separately for pilot injection and regular injection under such an engine operating condition as low rotation rate in order to cope with noise. However, under a high load, low rotation rate condition, it is suitable to permit the pilot injection to be made under low pressure and the regular injection under high pressure.